1. Field of the Invention
This invention relates to an image reading device, an image forming device, and an image reading method.
2. Description of the Related Art
FIG. 10 shows a reading optical system in an image reading device according to the related art.
The image reading device reads an image from a document by using the reading optical system.
In the reading optical system of FIG. 10, an image portion 2a of a document 2 placed on a contact glass 3 (which is a document base) is located near an optical-axis P of the reading optical system, and the image portion 2a is irradiated by a light beam from a light source 1z. A reflected light beam indicating the image portion 2a is led to a lens 14 via reflectors 15a, 15b and 15c, and the reflected light beam is focused on a light quantity sensor 4 through the lens 14. The light quantity sensor 4 is a solid-state image pickup device which is called a light receiving element, a CCD (charge-coupled device), a CCD element or an image sensor. A reflector 13 is disposed to effectively irradiate the document 2 by the light beam from the light source 1z. 
The image reading device is usually arranged so that a plane image (in two dimensions) of a document is read rather than reading a point image of a document. The image reading device is arranged to read image information from a document in the horizontal direction in FIG. 10. This direction is called the sub-scanning direction.
One method of reading the image in the sub-scanning direction is to read image information from the image portion 2a of the document 2 using the light quantity sensor 4, by moving the document 2 on the contact glass 3 in the sub-scanning direction, so that the image information is read serially in the sub-scanning direction. This method is called the image reading by automatic document feeding.
Another method of reading the image in the sub-scanning direction is to read image information from the image portion 2a of the document 2 using the light quantity sensor 4 in the sub-scanning direction, while the document 2 on the contact glass 3 is fixed. In this image reading method, the document 2 is fixed relative to the lens 14 and the light quantity sensor 4, and the module including the light source 1z, the reflector 15a and the reflector 13 is moved in the sub-scanning direction (or, in the direction from the left to the right in FIG. 10). The position of the document 2 which is irradiated by the light source 1z is moved in the direction from the left to the right in FIG. 10. Simultaneously, the optical axis P between the reflector 15a and the document 2 is moved in the direction from the left to the right in FIG. 10, and the intersection 2a of the document 2 and the optical axis P is always an image reading position of the light quantity sensor 4. In this way, even when the document is moved, the image information can be read from the document in the sub-scanning direction.
By moving the reflectors 15b and 15c in parallel with the reflector 15a at the speed equal to one half of the moving speed of the reflector 15a, the distance of the optical path from the document 2 to the lens 14 can be maintained at a constant value.
Thus, the ratio of the quantity of receiving light on the surface of the lens 14 to the quantity of irradiating light on the document 2 can be maintained at a constant value.
Next, a method of reading the image from the document in a main scanning direction will be described. The main scanning direction is the direction perpendicular to the drawing of FIG. 10, and this main scanning direction is perpendicular to the sub-scanning direction in which the light source and the reflector are moved relative to the document.
FIG. 11A shows the composition of a reduction optical system in the reading optical system of FIG. 10 which is viewed from the side of the sub-scanning direction.
For the sake of convenience, the illustration of the reduction optical system in FIG. 11A is modified as follow, the reflectors 15a, 15b and 15c are omitted and the position of the light source 1z is shifted to the position over the document 2, in order to express the actual distance of the optical path from the document 2 to the lens 14 in the reading optical system along the optical axis. Apparently, the distance of the optical path in FIG. 10 is enlarged in the vertical direction in FIG. 11A.
In the reading optical system shown in FIG. 11A, the image of the document 2 spreading in the main scanning direction is reduced by the lens 14, and the reflected light beams from the document 2 are focused on the line sensor part 4h by the lens 14. In the line sensor part 4h, plural light quantity sensors 4 are arrayed in parallel to the main scanning direction. The image information is read from the respective light beams focused on the line sensor part 4h by the respective light quantity sensors 4. For this reason, this reading optical system is called a reduction optical system. In the image reading device, the plane (two dimensions) image is read from the document in both the main scanning direction and the sub-scanning direction.
Theoretically, the image information in the main scanning direction can be read by the light quantity sensors without time lag. However, a certain time is needed to convert the read information into digital data, process the data and store the processed data. For this reason, in many cases, the two-dimensional image information is read serially by the image reading device, so that the process of reading easily accords with the data processing including the converting, the processing and the storing.
Examples of the light source used for irradiating a document may include rod-like lamp light sources, such as a xenon lamp, a cold cathode tube, and a halogen lamp. In many cases, the light source of this type is arranged to irradiate the whole length of the main scanning direction at a time. It is impossible to adjust partially the quantity of light in the light source of this type.
When the rod-like lamp light source is used, the quantities of the reflected light beams focused on the light receiving elements of the line sensor part may not be uniform in the main scanning direction. To avoid the problem, it is necessary to adjust the quantities of irradiating light by interrupting partially the irradiating light beams mechanically (mechanical shading), in order to obtain the required quantities of the irradiating light beams.
In recent years, a line-type LED light source in which plural LEDs (light emitting diodes) with high intensity are arrayed in parallel is adopted increasingly, rather than the rod-like lamp light source mentioned above.
In the case of the line-type LED light source, the light quantity of each LED is controllable, the light quantities of the respective LEDs in the longitudinal direction of the light source can be partially adjusted, and effective use of irradiating light is also possible.
Japanese Laid-Open Patent Application No. 2002-314760 discloses an image reading device including a reading optical system formed of a plurality of LEDs (light emitting diodes), in order to eliminate the nonuniformity of the irradiating light quantity and improve the quality of a reproduced image by reducing partially black image portions (shadow).
Japanese Laid-Open Patent Application No. 2006-245955 discloses a method of equalizing the distribution of the quantities of irradiating light to a document by using a reflector of a reading optical system in order to improve the ripples (uneven quantities of irradiating light) of the irradiating light to the document surface.
In the reduction optical system of the image reading device according to the related art, the angle of spreading of the irradiating light beams to the document 2 in the main scanning direction is comparatively large, and the angle of spreading of the reflected light beams incident to the lens 14 is also comparatively large. Generally, the lens 14 has the optical characteristic such that, when the angle between the optical axis and a light beam entering the lens 14 is comparatively large, the quantity of a transmitted light beam after passing the lens 14 is attenuated from the quantity of the light beam before entering the lens 14.
FIG. 11B is a diagram for explaining the optical characteristic of the lens 14. In the case of FIG. 11B, it is assumed that a document 2 having a uniform optical characteristic, such as a white reference plate, is irradiated by a uniform-quantity light from the light source, and a reduction optical system containing the lens 14 is used to focus a reflected light beam from the document 2 on the line sensor part 4h. FIG. 11B shows the quantities of the attenuated light beams by the lens 14 and the quantities of the receiving light beams on the respective light quantity sensors of the line sensor part 4h. 
As is evident from FIG. 11B, even when the quantity of light of the document 2 is irradiated by a uniform-quantity light from the light source, the quantities of the receiving light on the line sensor part 4h at end positions apart from the optical axis P are significantly reduced according to the light quantity attenuation characteristic of the lens 14.
In other words, the lens 14 in the reduction optical system has the optical characteristic such that, when the light reflected from the document 2 enters the lens 14 at an end position apart from the optical axis P, the quantity of transmitted light from the lens 14 is easily attenuated.
If a shading correction is performed on the output of each light quantity sensor 4 in the line sensor part 4h, the quantity of the receiving light can be corrected by electrically amplifying the output of each light quantity sensor 4 in a controlled manner. However, if a certain noise occurs before the photoelectric conversion of the output of each light quantity sensor 4 or the correction by the amplification of the output of each light quantity sensor 4, the S/N (signal/noise) ratio is increased after the photoelectric conversion or the correction, and the influence of the noise is enlarged.
Even if the read image information in this case shows a good quality (with a small S/N ratio) at the central portion of the document, the read image information shows a poor quality (with a large S/N ratio) at the end portions of the document which are apart from the optical axis. The quality of a reproduced image as a whole image becomes poor.
Conventionally, in order to reduce the variations of the image quality due to a lowering of the S/N ratio of the document end portions and to equalize of the receiving light quantities of the central portion and the end portions, a mechanical shading unit which partially shades a central light beam is used. However, even if the mechanical shading unit is used, it is difficult for the image reading device according to the related art to use the irradiating light of the light source effectively and equalize the receiving light quantity of the receiving light sensor over the whole light receiving surface.